Robert Boyle studies the propagation of sound in a vacuum. He uses a glass dome with the air pumped out. Inside the dome there is a hammer, which can be operated from outside. The hammer strikes a small metal bell and produces a sound. With the help of a valve, Boyle can adjust how much air is in the dome. He notes that the volume of the sound changes. He concludes, "that the air - be it the only medium or not - is at least the main medium for sound".